This invention relates to an aqueous process for producing soy protein concentrates and, more particularly, to concentrates that possess many of the functional properties of soy protein isolates.
Although food-grade soybean flours and grits have been available since the 1930's, it has been only within the last twenty years that soy protein products have been used at an accelerating rate for the manufacture of processed food. During this period, there has been a proliferation of improved products, including soy protein concentrates (minimum 70% protein), isolates (minimum 90% protein), and textured products.
In most instances, the nutritional quality of a protein ingredient for food is a vital factor. However, in many cases, the functional properties of the ingredient is an over-riding factor since the product must contribute to, or at least not detract from the overall character of the food being processed. This is true of both conventional foods and newly designed foods. All foods, old and new, must conform to some prevalent pattern of consumer acceptability in order to achieve successful commercialization.
In the food field, functionality is a term which refers, in a general sense, to the property or properties of a food ingredient or additive that defines or influences the character of any processed food containing the ingredient or additive. For example, the milk protein or soy protein stabilizes emulsified fat in a whipped topping, and also stabilizes the foam structure when air is incorporated through whipping. The newer soy proteins have been selected and characterized for functional use in processed food items.
The functional character of soy proteins includes properties such as solubility, water adsorption and holding capacity, fat absorption and holding, emulsification, viscosity, gelation, cohesion-adhesion, foaming, flavor binding, and the like. The development of soy protein products with new or, particularly, improved functional properties, has been an ever-increasing challenge.
Soy protein concentrates have been defined as products prepared from defatted soybean source material by removing a preponderance of soluble non-proteinaceous material, and containing a minimum of 70 percent protein (N.times.6.25) on a moisture-free basis. These products have been commercially available since about 1960.
Prior to the commercial introduction of soy protein concentrates, the only soy protein products available for food use were soy flours and grits (about 50% protein). Concentrates were designed to offer improved nutritional value (higher protein content) and improved functional values, including flavor and odor.
Three basic commercial processes were developed during the late 1950's and early 1960's to provide these concentrates. The common element in these processes is the immobilization of the major protein fraction of the defatted soybean source material in aqueous suspension to permit the removal of soluble low molecular weight materials including sugars, non-protein nitrogenous matter, some minerals, and the like.
In one process, as described in U.S. Pat. No. 2,881,076, acidification of the aqueous solution to the average isoelectric pH of the protein is utilized to immobilize the protein. The low molecular weight materials are then removed by aqueous leaching. In this process, the wet acidic concentrate may be neutralized with food-grade alkali prior to drying. This improves the solubility of the protein.
In another process, the defatted soybean source material is leached with aqueous ethanol to remove the low molecular weight materials (Mustakas, Kirk, and Griffin, J. Am. Oil Chem. Soc. 39, 222, 1962; U.S. Pat. No. 3,365,440). In the third basic process, the protein of the defatted source material is immobilized by steaming prior to the leaching with water (U.S. Pat. No. 3,142,571). Since the early 1960's, modifications of several of these processes have been proposed or commercialized.
The soy protein concentrates possess, in some degree, a number of the functional properties recited above. The kind and degree of these functional properties are dictated by the processing parameters. For example, the aqueous-alcohol process concentrates possess low protein solubility in spite of the fact that their aqueous suspensions are almost neutral in pH. This is a result of protein denaturation by aqueous alcohol and heat in desolventizing. In contrast, the acid-leach concentrates which are neutralized with alkali possess a higher protein solubility.
Although the functional properties of soy protein concentrates are useful in the production of certain processed or manufactured foods, this is a limiting factor in the broader application of these concentrates in the food field. They suffer by comparison to the soy protein isolates as to kind, range, and degree of functional activity. Thus, there has existed a distinct challenge to improve the functional properties of the concentrates to allow replacement of the more expensive isolates in processed foods. All soy protein products have been increasing in cost because of inflationary economic factors, yet it is expected that the differential between concentrates and isolates will continue to widen due to the inherent complexities of isolate processing.
It is apparent that there is a need for soy protein concentrates with greater functionality produced at lower cost relative to the cost of producing soy protein isolates.